1. Field of the Invention
The invention relates to an exhaust catalyst temperature estimating apparatus and an exhaust catalyst temperature estimation method for an internal combustion engine.
2. Description of the Related Art
An exhaust catalyst for purifying exhaust gas is arranged in the exhaust system of internal combustion engines. The catalyst loaded in this type of exhaust catalyst has an activation temperature, and is unable to adequately purify exhaust gas if the temperature of the catalyst is equal to or below the activation temperature. As a result, it is necessary to determine whether the catalyst has reached the activation temperature by estimating the exhaust catalyst temperature as a catalyst temperature.
For example, Japanese Patent Application Publication No. 5-248227 (JP-A-5-248227) describes estimating current exhaust catalyst temperature by identifying a model formula for estimating current exhaust catalyst temperature that uses current engine speed, current fuel injection volume and current combustion air-fuel ratio as variables by applying to an actual engine through compliance testing, and then using the model formula to estimate current exhaust catalyst temperature.
By estimating exhaust catalyst temperature as described above, the identified model formula also takes into consideration the generation of heat by unburned fuel and the like contained in the exhaust gas due to a chemical reaction in the exhaust catalyst. However, there is no guarantee that a chemical reaction generating the same amount of heat will occur at all times, even under identical conditions. In addition, if the catalyst deteriorates, the amount of heat generated from the chemical reaction under identical conditions steadily decreases. Thus, even if exhaust catalyst temperature is estimated using a model formula that takes chemical reactions into consideration, the estimated exhaust catalyst temperature may exceed the actual temperature, which may result in incorrectly determining that the catalyst has reached the activation temperature.